text:
Is the turning error of a magnetic compass greatest when tuning through South in the Southern Hemisphere?
Question:
The turning error of a magnetic compass in the Southern Hemisphere is greatest when turning through South.
Is the statement true or false?
And why is this/isn't this the case?

This sounds like a homework question. What research have you done?

This is from an online question database that I'm using to study. This is from Wikipedia: The compass dial will tend to align itself with the geomagnetic field and dip toward the northern magnetic pole when in the northern hemisphere, or toward the southern magnetic pole when in the southern hemisphere.This is about the best explanation I could find. I am still confused.

I have also been using the FAA Instruments manual

To @GdD's point, when you post this sort of question, make it clear that it's a question from some quiz or test and you have thought it through and done some research, and you are stumped and need help.  That mollifies the folks here who can get annoyed by students too lazy to do their own research, which is the assumption when a question is posted verbatim with no extra information.

“The turning error of a magnetic compass in the Southern Hemisphere is greatest when turning through South. Is the statement true or false?”
True
I am going to answer this question for a second time and delete the first try. It was a mess. Hopefully, this one will make more sense even though it has the same information. Just a better structure.
There are three factors that would explain why turning errors are greatest when turning through North and South. They are specific to the aviation wet, liquid-filled, or whiskey compass. Officially, two of them are magnetic dip and acceleration errors caused by magnetic dip. Unofficially, the final one is the counterintuitive way that you have to read the compass.
The magnetic compass turning errors that we usually identify as UNOS and ANDS while in the Northern hemisphere are caused by a combination of magnetic dip and the off center balancing of magnetic dip. As you have stated in your comment, a compass has the tendency to dip it’s North-seeking element down towards the Earth in the Northern hemisphere. The same is true for the South-seeking element in the Southern Hemisphere. The Northerly turning errors (UNOS) are greatest (up to the same amount of error as your degree of latitude) when turning through North or South.
In the picture below, the black text is the actual direction to which you are turning. The red text is the amount of compass error at 30° latitude.

An aviation, liquid filled, “whiskey” compass works the same way as a handheld lensatic compass. It has a magnetized bar held in place on a low friction bearing, needle or pin point. In the case of the whiskey compass, there is also a card noting the bars direction attached to it that is read counterintuitively.
To better understand this, remember, with a whiskey compass (as opposed to a dry, vertical card, compass) you are reading the directional card from the opposite side of the actual direction that you are facing. You read the North direction from the end of the side facing South, and the East direction from the end of the side facing West. Picture it like the lid of a bottle or can. If you are looking at it from above, you can mark “N” at the edge of the top closest to North. If you are looking at it from the side, you can not see the top. So, you put an “N” on the side furthest away from North because that is the only side you are able to see when facing North. The same goes for the other directions. Therefore, East is physically to the left of North (clockwise when viewed from the top) on the direction card. The numbers of degrees is read in increasing increments from right to left. A good mnemonic memory aid for this is, “If you always run/turn away from the one you want, they will chase you.”
The magnetized bar will always try to align itself or stay aligned with the magnetic flux lines of the Earth. The Earth is a giant magnet. The flux lines Of the Earth exit the Earth at its magnetic South Pole and re-enter it at its magnetic North Pole all the way around the Earth. This places the flux lines comparatively horizontal (parallel to the ground) at roughly the equator, and vertical (perpendicular to the ground) at either Pole. The closer you get to either Pole (the higher the latitude), the more the compass bar will try to align itself vertically.
To counteract this phenomena, compasses are sometimes weighted to move the center of gravity of the bar & card arrangement closer to the side of the bar that would point upward, away from the Earth. In the Northern Hemisphere, the CG would favor the magnetic bar’s Southern pole. In the Southern Hemisphere, the CG would favor the magnetic bar’s Northern pole. This is done so that the bar or anything attached to it will not touch the inside of the compass housing. Any type of contact, friction, or drag could stop the compass from moving and realigning itself freely. When the compass is not weighted to move its CG, a compass that is suspended from its top will move its CG towards the opposite direction of the compass’s magnetic dip on its own.
The CG of the compass’s moving parts is very important in its movement. It creates inertia. The inertia of the offset CG will be apparent whenever there is a change in speed or a change in direction of movement. The CG will always want to remain at the same speed and travel in the same direction as it was. The CG will want to lag behind during acceleration, keep moving forward during deceleration, and keep going straight, swinging to the outside of a turn/curve. Picture it as the same phenomenon that will cause a glass marble in a Petri dish to roll around due to velocity change. Changing speed and/or direction will cause the compass to become misaligned until speed and direction are stabilized. When facing North in the Northern hemisphere, a right hand turn to the East will cause the compass to initially spin clockwise (relative motion to the aircraft) to the West backside indication. Here is a good article on turning errors that explains it much better than I can. Or, this one from AOPA.
In the case of a compass where an off-center CG is not affecting the compass, aircraft banking will cause magnetic dip to swing the directional card out of alignment. When facing North, banking to the right (East) will cause the compass to initially spin clockwise (relative motion to the aircraft) to the West backside indication. Due to this directional indication from the perspective of the backside of the directional card, magnetic dip while in a coordinated bank (where g-forces keep the weight of all objects inside the cockpit acting perpendicular to the floor) would cause the compass to initially indicate a turn in the opposite direction of the actual physical turn of the aircraft if your direction is North in the Northern Hemisphere and South in the Southern Hemisphere. This means that the compass would initially spin clockwise in the above North to East turn. The compass would then lag the actual turn until the bank is brought back to level, or your aircraft reaches an East or West heading. This article, as well as this one here both explain the above phenomenon better.

I tried to extract an answer to the specific question given, but I guess I have to read through again

@Jpe61 - Yes, I know. Parts of VDMONA are more of a take it on faith sort of thing rather than a understand the details sort of thing. Way too complex for my little brain. Most importantly, it’s counter-intuitive due to the perspective of the wet compass to the pilot pilot being reversed. When you are facing North, you are looking at the South end of the compass even though the South end is marked as North. The same reversal goes for East and West.

With all the weird stuff related to magnetic compasses, I've come to a conclusion that they are only good for learning all the weird stuff about them. For navigation, nope!

Just be aware that the southern MAGNETIC hemisphere does not exactly coincide with the southern hemisphere.  Google it for more.

@Jpe61 -- yet I actually had an experience when cloud-flying in a hang glider (location, even nation, will go un-named) with a turn rate indicator as my only gyro instrument, when I felt I was high enough (cold enough, frost starting to form on eyeglasses) and was ready to try to follow a straight course to leave the thermal updraft, and I had better luck capturing a heading and avoiding further turns by following the mag. compass (on an optimal heading for the location) than by following the (non-aviation) GPS, which had too much lag to be useful, given the high turn rate characteristic of a

(ctd) slow-flying aircraft at even moderate bank angles.  If the heading is optimized for the location (magnetic hemisphere), the lead-lag issues can actually work in the pilot's favor and give an indication of a developing bank and turn even before the heading has changed very much.  Because bank is sensed, as well as heading.  Just as a turn "coordinator", by sensing roll rate as well as yaw rate, can give a quicker indication of a developing bank and turn than a turn rate indicator.  Each instrument has its own unique strengths and weaknesses, and it is good to understand what they are.

I do agree


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What happens to the high pressure below an airfoil when the angle of attack increases?
I recently came across this question in a principles of flight exam:
When the angle of attack of an airfoil increases

the high pressure below the airfoil increases, the induced drag increases
the high pressure below the airfoil decreases
the induced drag decreases
can't remember this option

I chose 1. as the correct answer and it looks like I got it wrong. Although, I cannot tell for sure.
I know the induced drag will increase, but will the high pressure underneath the airfoil increase or decrease with an increase in angle of attack?

What makes you think you were wrong?

What is held constant?  Airspeed, or lift force?  Some the answers below make an assumption that airspeed is held constant and lift force is not, which may not be justified.  But I suspect the answer would be (a) in either case.

We need to find out what 4. was, 1. seems correct.  2. and 3. are incorrect.

@RobinBennett when I got my report back, that question number was marked incorrect. The description on the report was "lift and airfoils" and it was one of only 2 Principles of Flight questions.

@quiet flyer - good point, if you wanted to continue in level flight with an increased AOA, you'd have to reduce speed. The pressure would stay the same, and I'm not sure what the induced drag would do... It seems like a big assumption to me though.

Your answer is correct. The pressure distribution from the xfoil simulation on a NACA 0012 are shown below for AOA 0 and AOA 5. As you can see, the lower side pressure does increase. However, most of the lift is generated by the suction on the upper surface, which is typical of subsonic flight.

A third graph from 12 degrees AOA would be great.  This may show the less efficient "bottom" lift, with its proportionally larger induced drag adding to the Coefficient of Lift (near stall).  The polars of the NACA 0012 show max Clift to Cdrag at around 6-7 degrees AOA.

Thanks for the explanation!

I'm pretty sure you were right. More angle of attack means more lift. More lift is caused by higher pressure under the wing and lower pressure on top. More lift causes more induced drag.

Yes, I agree there and that was my train of thought...there are many known mistakes in the exams database so I would not be surprised if it is a mistake.

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What causes lift?
Now this seems to be the question of the ages, and various questions on this site regards it.
The often told description of lift is that it is due to air traveling faster over the wing than under it, creating pressure differences according to the Bernoulli equation.
This however, makes little sense to me and as I've read it is entirely false. Particularly interesting is this article where the author writes that

Those with this view must also believe that one can pinch with one finger and clap with one hand.

I had settled with the description that lift is caused by the downwash of air which imparts an equal but opposite force on the wing itself. As noted in the linked article, the downwash is attributed to the impact of air on the lower side of the wing constituting some, albeit little of the lift, and the air on top of the wing being curved downwards following the coanda effect.
Now, my confusion was re-initiated as I am currently reading "Aircraft Design: A Conceptual Approach" 6th edition, by Daniel P. Raymer - a highly regarded aerospace engineer.
In the 4th chapter on airfoil and wings, he writes:

An airfoil generates lift by changing the velocity of the air passing over and under itself. The airfoil angle of attack and/or camber causes the air over the top of the wing to travel faster than the air beneath the wing. Bernoulli's equation shows that the higher velocities produce lower pressures, so that the upper surface of the airfoil tends to be pulled upward by the lower-than-ambient-pressures while the lower surface of the airfoil tends to be pushed upward by the higher-than-ambient pressures.

Now I cannot imagine Raymer to have any misunderstanding on what causes lift, which makes me very curious about this statement.
The pressure difference found in calculations by using the Bernoulli equation and an equal transit time is much insufficient to lift an aircraft and windtunnel experiments show that the air on the top of the wing reaches the trailing edge much sooner than the air traveling below the wing.
Is it a simplification to avoid becoming too technical and into the "physics" for the average reader - or is something else going on? It is very difficult to become confident in what actually causes lift with a seemingly endless number of descriptions.
As a side note: on the bottom of the page in the book, this common dispute is mentioned and told that this view of lift is completely correct as is the Newtonian downwash description.

Ultimately lift is the equal and opposite force generated by deflecting air downward.  And consider a wing with a lower surface at an angle to the relative wind and a flat top surface exactly parallel with the relative wind.

Consider a paraglider wing - if the Bernoulli effect did’t provide significant lift on the upper surface then the wing wouldn’t stay in shape.  The forces on the upper and lower surfaces are well understood; not liking the physics won’t make it go away.

Does this answer your question? How do wings generate lift?

Recent (2020) write-up in the Scientific American "No One Can Explain Why Planes Stay in the Air", including references to the most relevant sources:
https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/

Money. This is actually an aeronautical engineering joke I learned before transferring out of aeronautical engineering as a freshman 52 years ago and represents the only thing I took with me from that field.

Heavens, we can't talk about lift around here.  Yes!  Close immediately!

@Wasserwaage That article is a disgrace to Scientific American. It simply claims we could not explain Bernoulli's law when we actually can (conservation of energy). Being a private pilot himself, I guess the author simply listened too much to the "expert" opinion of other hobby pilots and their uneducated drivel.

Just because someone has access to an editor and a webpage does not mean that they understand the topic they write about.
Short answer: Raymer and Bernoulli are right. The finger-pinching / hand-clapping comparison is silly.
When a wing approaches at subsonic speed, the low pressure area over its upper surface will suck in air ahead of it. See it this way: Above and downstream of a packet of air we have less bouncing of molecules (= less pressure), and now the undiminished bouncing of the air below and upstream of that packet will push its air molecules upwards and towards that wing. The packet of air will rise and accelerate towards the wing and be sucked into that low pressure area. Due to the acceleration, the packet will be stretched lengthwise and its pressure drops in sync with it picking up speed. Spreading happens in flow direction - the packet is distorted and stretched lengthwise, but contracts in the direction orthogonally to the flow. This contraction, by the way, is necessary so the air can move out of the way of the wing. The thickness of the wing alone already causes this acceleration and stretching on both sides of a symmetric airfoil at zero angle of attack. With increasing angle of attack and/or airfoil camber, the flow around the wing becomes asymmetric and more acceleration will happen on the suction side of the wing.
Once there, the air molecules will "see" that the wing below them curves away from their path of travel, and if that path would remain unchanged, a vacuum between the wing and our packet of air would form. Reluctantly (because it has mass and, therefore, inertia), the packet will change course and follow the wing's contour. This requires even lower pressure, to make the molecules overcome their inertia and change direction. This fast-flowing, low-pressure air will in turn suck in new air ahead and below of it, will go on to decelerate and regain its old pressure over the rear half of the wing, and will flow off with its new flow direction. You see, as Bernoulli found out, faster air has less static pressure and the speed difference between both sides creates a pressure difference. Integrated over the wing surface this is lift.
Note that lift can only happen if the upper contour of the wing will slope downwards and away from the initial path of the air flowing around the wing's leading edge. This could either be camber or angle of attack - both will have the same effect. Since camber allows for a gradual change of the contour, it is more efficient than angle of attack.
With the changed direction of air flow the air is leaving the wing at an angle compared to its initial direction. This change of direction imparts a momentum on the air which, according to Newton's third law, needs an opposite force, which is lift. You see, lift can be explained in different ways and all of them describe in the end the same process.

I submit that the classic Bernoulli experiment with a glass u tube simply shows moving fluid acts as a vacuum pump, much the same way as a sink top aspirator.  This actually agrees with the Coanda effect.  The only reason the moving stream has lower pressure is that where it is going has even lower pressure.  Otherwise the air would not be moving.  The moving fluid drags some of the static fluid (air) along with it.  With wings, energy for this "pump" is provided by ... thrust!

There's a few things here I don't understand. First as you say, the air accelerates towards a low pressure area on top of the wing. It appears from your explanation, that this "initial" low pressure is not due to the vacuum forming by following the curvature of the wing. The acceleration causes pressure to lower, and more air is drawn in. But what exactly causes this low pressure to begin with? Perhaps due to the wing thickness if I understand correctly?

@Erik There is only one low pressure area. It is caused both by thickness and by the surface curving away from the initial flow direction. In subsonic flow all transitions are gradual, so the low pressure makes itself felt already ahead of the wing, but there is no "initial" low pressure.

I'm a little late to the party, but I wanted to know if what I thought about the second paragraph of your answer is correct, so here's what I've gathered : The air will follow the contour of the wing because where the wing starts to curve away, a low pressure is created, therefore getting filled by the air that wasn't going to follow the contour. (If that doesn't make sense, I can try to convey what I'm saying a little more)

@Wyatt Always keep the equilibrium between inertial, pressure and viscous forces in mind. If the air would not follow the wing, a vacuum would appear. If it follows the wing faithfully, no pressure gradient would exist. What happens is something in between, where inertial and pressure forces are in an equilibrium.

@PeterKämpf I see, that makes sense. I thought there would be some sort of medium where the pressures kind of level out. Thanks!

Both Bernouilli and Newton indeed happen in subsonic airflow over wing profiles, but Newton provides a much better picture:

Bernouilli describes incompressible flow, yet wings still provide lift in high subsonic and supersonic circumstances.
A flat plate still provides lift when at an angle of attack other than zero, despite Bernouilli not being present.

In the flat plate case there is higher pressure at the bottom (at AoA > 0) and lower on top - due to the sharp nose the air cannot follow the surface immediately, yet there is still some underpressure in that region.
My choice: Newton.

Several places I've read, as well as the article I linked tell states that the static pressure term in the Bernoulli equation disappears for unconfined flows such as the flow over a wing because it assumes the atmospheric value for pressure. How can Bernoulli be applied in this case? Is it truly applicable to this case, or does it only provide means to calculate the pressure from the change in velocity from an energy point of view?

It's indeed a source of confusion. The static pressure at infinity is the atmospheric pressure indeed, not the local static pressure. Bernoulli needs to be considered in a stream tube. I reckon Bernoulli provides a means of explanation for starter pilots, that's it. Too many inconsistencies and invalidating circumstances.

Why should Bernoulli not "be present" on flat plates or with compressible flow? It does cover all those cases, too.

@PeterKämpf If I recall correctly, Bernoulli is derived based on energy. So when the flow picks up speed and dynamic pressure, the static pressure should drop due to conservation of energy. It would seem intuitive that the surrounding atmospheric pressure would "violate" this and add the extra energy to the flow, keeping the static pressure constant. Even locally. Now it seems I am misunderstanding something regarding this that I don't understand. Does the local conditions "win" over the atmospheric pressure in the short timeframe the air takes to travel the length of the wing?

@Erik well, if you're traveling at 30 m/s with a 1.3 m wing, "transit time" is a fraction of a second, not enough time for the deflected airstream to return completely to the wing surface.  Local conditions "win".  An easy to see fluid example is at the transom of a boat.  When it is slow, the water "sticks" to it, but once the boat speeds up, it separates, and the shape of the fluid flow wake is no longer determined by the shape of the transom, but by the fluid "chasing" the low pressure area behind the boat, forming the classic "V".

Air hits the wing. Air hits every square inch of the surface of the aircraft. And it is hitting every square inch of the surface all the time, even when the aircraft is not moving, when there is no relative wind. Air molecules are always moving (Brownian Motion). And because they are always moving, they are always hitting the surface of the aircraft. Every time an air molecule hits and bounces off the surface of the airframe, there is an exchange of momentum (a.k.a. a FORCE) and this force is applied to the surface, perpendicular to (or Normal) to the surface. The total aerodynamic force on the airframe is just the vectorial sum of all these forces caused by all the molecules hitting and bouncing off the airframe.
Lift, is just the component of this total aerodynamic force that lies normal (or perpendicular) to the flight path of the aircraft through the air. Why is the distribution of Lift uneven? because the momentum exchanged when an air molecule hits the wing is dependent on the velocity of the air molecule and the angle at which it is moving locally relative to that surface.
Drag is just the component that lies parallel to the flight path.
An air molecule hitting the very back tip of a ICBM reentering the atmosphere at Mach 20 is still pushing the missile forward.... But the air molecule at the front of the ICBM is hitting the nose with a much greater velocity - More velocity, bigger momentum change, greater force.
All the other explanations are just aggregate simplifications used to make calculations and help explain complex side effects of aerodynamic forces on specific airfoil shapes, (or observations of aggregate effects like wingtip vortices, or downwash, etc. etc. etc., which are necessary to conform to principles of conservation of Energy or Momentum).
The Bernoulli explanation is especially silly. How then do Symmetrical airfoils create lift? Golly, How do aircraft generate negative Lift and fly upside down? The increase in speed on the top of the wing is a side effect of the lift, not the cause. The underlying justification for the Bernoulli explanation is called the "equal Transit time" theory, that any air molecule traveling over the top of the wing must transit a longer distance in the same amount of time as a molecule transiting the bottom of the wing. This has been shown to be false in wind tunnel tests.
If you're interested, you can watch a more detailed academic presentation of why both Bernoulli and Newtons "downwash" explanation are factually incomplete, and only represent an explanation of the consequences of LIFT, not an explanation of the causes of lift. It's available on youTube at:
Common Misconceptions in Aerodynamics
NOTE. I use the word "hitting" with some literary license. The molecules never actually "hit" anything, they just get close enough for the electromagnetic forces exerted by their electron clouds to repel one another.

Symmetric airfoils generate lift with a non-zero angle of attack. This can be very nicely seen for example in the CL vs alpha graphs here: http://airfoiltools.com/airfoil/details?airfoil=naca0015-il. Additionally, your explanation with air molecules can be very nicely extrapolated to arrive at Bernoullis principle. Your explanation is therefore the same as explaining it with that principle...

If by "Explaining", you mean to attribute causality, then no, you cannot so extrapolate. correlation is NOT the same as causation.  Almost every heroin addict drank milk as a child...   And extrapolation or derivation  is also different from attributing causality. There are many, many aggregate properties of fluids that must be as they are because of Conservation of Energy, Momentum, and/or principles of Symmetry, but that does not justify using them to explain or attribute one as the cause of the other.

Re "The Bernoulli explanation is especially silly. How then do Symmetrical airfoils create lift?"  -- no, when you look at the flow around a symmetrical airfoil flying at positive angle-of-attack, you can see how differences in flow velocity are created, leading to differences in pressure.

Re " they just get close enough for the electromagnetic forces exerted by their electron clouds to repel one another."-- this answer might benefit from some discussion of the "boundary layer"-- the unimpaired free-stream flow is surely not "hitting" the wing (or getting close enough for molecules to be repelled by electromagnetic forces exerted by electron clouds)

Perhaps, but the "free-stream flow" has nothing to do with lift. It is as you said never in contact with the aircraft. How can it exert a force on something it never comes in contact with. All these "explanations" are, as I said, only observations about many-times removed indirect consequences of energy and/or momentum conservation effects.

@quietFlyer, Also, How can be certain that the difference in velocity is causing the pressure difference, and not the other way around? In fact, it seems to be that changing the pressure is always certain to increase velocity, all other things being equal. (dynamic pressure is defined by velocity - that's why we use pitot tubes to measure velocity). And sub-sonically at least, total pressure is constant, that's the essence of Bernoulli - conservation of energy. As speed increases, dynamic pressure increases, static pressure decreases. Their vector sum remains constant

Welcome to the club.  What happens on the bottom of the wing is fairly straight-forward action reaction, also seen with water skis or sticking your hand out the window of a moving car.
The "top lift" is a little tougher to explain and actually only happens under certain conditions pertaining to Reynolds number.  Top lift is desirable because it comes with very little drag expense.  Effects of Reynolds number with various airfoils can be studied at www.airfoiltools.com.
Just remember, the wing hits the air, but is explained in a relative sense as an "airstream" (also true in wind tunnels).  Coanda may be a better explanation of top lift than Bernoulli.
Using the "airstream" model, consider that air molecules have mass, and if the flow is fast enough, their momentum cannot quite follow the curve of the wing, causing pressure to drop near the upper wing surface.  This is Coanda!.  A little like a bunch of race cars approaching a turn.  There will be a void of cars near the inside of the turn.
Remember, that "vacuum bubble" on top is only possible with adequate chord and airspeed, and will collapse (with massive increase in drag) if proper Angle of Attack is not maintained.  This is the "stall" that early aviators, in their slow, light aircraft, experienced.

Interesting description where the air momentum cannot follow the curvature of the upper surface. As you say, Coanda is when the vacuum is created from the air not being able to follow the curvature. How come then, that water flowing from a faucet will follow an included glass below it? Are there different effects taking place?

Water molecules have a much stronger attraction to each other than air molecules.  This is one reason why water is a liquid at ambient Temps, while N2 and O2 are gasses.  And water is also attracted to glass.  Also try water at 100 km/hour. (Reynolds).

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How can adverse maneuverability due to a fuel tank behind the rear pressure bulkhead be mitigated in Airbus ZeroE hydrogen plane?
Airbus recently announced three concept aircraft for commercial use, powered by liquid hydrogen, with an ambition to have the plane in operation by 2035.
https://www.airbus.com/innovation/zero-emission/hydrogen/zeroe.html
https://www.airbus.com/newsroom/stories/these-new-Airbus-concept-aircraft-have-one-thing-in-common.html
From what I can tell, the turbofan variant is in the same segment as the current A320, albeit with somewhat reduced range.
Considering hydrogen powered planes, the challenge is to carry enough fuel to make it compete with kerosene.
There are several options, and from this question it seems 3 options stand out:

Cryogenic: storing the fuel as a liquid
Compressed: requiring heavy and strong fuel tanks
Hydrides: bind hydrogen with another substance and release upon heating

Airbus intends to go with option number 1, that is storing the fuel liquid and cryogenically behind the rear pressure bulkhead.
The unbalanced weight distribution storing not only the fuel, but assuming also heavy tanks and cooling equipment in the far rear of the plane seems like a maneuverability issue to me, with a pitch-up moment from the shifted weight balance.
What measures can be taken to mitigate the (assumed) adverse handling this creates?

Why assume that there will be any adverse handling? It looks to me like the fuel tank will be quite close to the centre of gravity.

Behind the rear pressure bulkhead would be pretty much as far back as you can go insida a plane. So pretty much as far as you can go from CoG.

@jpe61 My bad. I was thinking of the blended wing design where wing and engines are all behind the CG.

@Federico Please give questioners more time to refine their question to your taste before closing them next time!

Questions should be closed until such changes are made, not left open (attracting answers which will no longer be applicable). Closed questions can be edited; closing and reopening is better than deletion of multiple answers later.

@Nij while I agree with your point here, I'd argue that waiting a few minutes to see if an edit can be quickly put in place is favorable over closing a question and possibly waiting hours to have it reopened by the voting system

If it can be closed quickly, it can be opened quickly. It won't totally disappear in the hour or five that waiting for a mod flag needs, either.

@Nij I personally think it's worthwhile to wait a couple of minutes for an amended question, rather than having to possibly wait a few hours for a mod flag on a recently posted question.

Airbus will store hydrogen by chemically binding it to carbon for a looong time. This way, it can be put into the wings, a belly tank and the horizontal tail and its gradual disappearance during the flight will not shift the center of gravity much.
What the PR department just published is just for political correctness. It has not been thoroughly checked for practicability by the engineering department. Nobody seriously expects to use hydrogen for airliners. The problems of storage are obviously putting it at a huge disadvantage, and your correctly identified issue of storing fuel far from the center of gravity is one of those problems.
When an Airbus is delivered today, its tanks are filled with a blend  containing hydrocarbons from a carbon neutral source. Beluga flights delivering parts between the Airbus plants use this fuel. Virgin Airlines tested a renewable fuel already in 2008 and Lufthansa flights departing from Frankfurt also use it.
Of course, most airlines will refill with regular, mineral oil based kerosene after this fuel has been used up because it is cheaper and availability is much better. But technically there is nothing holding back airlines today to use a carbon-neutral fuel.
Generating hydrogen today is mostly done using natural gas which is not quite as carbon intensive as kerosene but everything but carbon-neutral. Also, the energy source for refining and transporting fuel from renewable sources will make them less than 100% carbon neutral. The current sources for renewable fuels (plant oils and waste materials) are less than optimal and hard to scale up, but work on using direct sequestration from atmospheric carbon dioxide is progressing. Today, we waste excess solar power by shorting it to ground. I know better uses for that excess energy …

Interesting. You say that Airbus intend to chemically bind hydrogen to carbon, why is this and do you have a source for this information? How does this statement compare to that noone will seriously intend to use hydrogen for aviation - they seem to counter each other? And will the carbon bounded hydrogen be a liquid fuel or a cryogenic fuel?

You omit the chief benefit from the natural gas to hydrogen conversion: it's feasible to capture and sequester the CO2. That's a bit harder when the CO2 is produced 30.000 feet in the air.

@Erik: In case the joke elided you, all fossil fuels are made out of carbon and hydrogen, typically a chain of 8-12 carbon atoms with hydrogen atoms attached. There's a small part where it's accurate; typically the carbon chains found in nature are a bit longer so some of these long chains are "cracked" in refineries. This involves adding hydrogen atoms at the point where the chains are broken.

@MSalters I'm not particularly proficient in chemistry, but what you and Peter are saying about "Airbus will store hydrogen by chemically binding it to carbon for a looong time.", is regular jetfuel? And this announcement from Airbus is nothing more than a dream?

@Erik: Yes, what I meant to say is that all airliners will continue to use kerosene (a mixture of molecules consisting mainly of carbon and hydrogen). That should not be misunderstood that Airbus personnel will be adding each hydrogen atom to kerosene molecules but that Airbus is working with companies which try to sequester carbon from the atmosphere and add hydrogen, ideally from water. Or leave this process to plants which do the same since maybe 2½ billennia.

@MSalters Absolutely correct. And then this carbon dioxide goes into fizzy drinks or enriches the atmosphere of greenhouses. In all cases it will eventually escape into the atmosphere. The amount that ends up in calcium carbonate is negligible.

@PeterKämpf So taking carbon from the atmosphere and adding hydrogen is to make synthetic kerosene? But then this talk about cryogenics and liquid hydrogen, in the shape Airbus talked about it at their press event, is false? If this is the case, wouldn't this quickly be uncovered by those with knowledge of aircraft design and looked at as a joke?

@PeterKämpf: I know that the Netherlands plans to store it in empty gasfields under the North Sea. Those have kept CH4 in for 100 million years, and CO2 is a bigger molecule. So this is indeed a feasible way to produce H2 from CH4 without leaking CO2 into the atmosphere.

@Erik: You have never worked in/with marketing, have you? This is Airbus trying to look politically correct. It is not a blatant lie, more like what you say about controversial topics without getting into hot water. Of course the engineers don't take that seriously.

@Erik The press release isn't false. Of course every option needs to be looked at, and at this stage this should happen with universities in the scope of master thesis or maybe even PhD programs, sponsored by Airbus. But that does not mean that now the thousands of engineers it needs to design an aircraft are working on a hydrogen powered airliner. That is not what the announcement says, either.

@PeterKämpf But Airbus do state in their release that they intend to have a functional hydrogen powered airliner in service by 2035, isn’t that pretty much the same?

@PeterKämpf are you sure " engineers don't take that seriously"? In Germany there are many research institutes on this (and it is not the only country doing this). Probably this will not succeed, but also the B747 was designed as a failing project. There is uncertainty of oil prices in future (and wars), so it is a normal diversification (to bring back projects in case of need).

@Erik: Any prognosis that concerns the future is especially hard. By 2035 all those people in responsibility for that intention will be in retirement, and as you know, intentions aren't permanent. This is only speculation on my part, but backed up by hard evidence concerning the technical feasibility. I really do think that this intention is not serious (or made by especially naive people, a less likely possibility in my mind). Just look how long we have been promised blended wing bodies (BWBs - one of the concepts). 30 years at least, maybe more. And what did we get? More of the same.

The situation is not as dire as one might think, as the energy content per kilogram of hydrogen is 3.3 times larger than that of kerosene (142 vs 43 MJ/kg):

Wikipedia: Energy Density -    Energy density in energy storage and in fuel
Taking into account a slight weight saving in wings, and a little heavier hydrogen tank and fuel lines (insulation, pressure), the weight of the hydrogen fuel itself and the fuel system  should be somewhere between half to third of the weight of a "traditional" fuel & fuel system.
It is true, however, that placing a constantly decreasing weight as far from the center of lift as possible, is a dumb idea. This would lead to very uneconomical trim condition at some point of the flight, unless the plane carries a movable trim weight, which also is kind of dumb idea IMO. Variable geometry wings might also be a solution, but again, would add weight and complexity.
I have to say that I think Airbus is pulling our leg here. It is not uncommon for companies to purposefully mislead their rivals in this way. I bet most of us are familiar with the many weird patents that emerge every now and then, the stupidest I can recall being slicing the headphone plug in half to save space in phones. Duh...

Yes, these designs are as speculative and fluid as SpaceX's were in 2003. BWB, seriously?!  "Airbus will need to launch the ZEROe aircraft programme by 2025."  This press release expresses interest in hydrogen, nothing more.  We can't reverse engineer something that hasn't been engineered yet.  We can only note the obstacles.

I recall reading long ago that the really big obstacle to a hydrogen fuel economy is the existing petroleum distribution system based on steel for tanks and pipelines, which can't be used for hydrogen distribution due to embrittlement.

Interesting angle about leg-pulling. There is/was something called "Airbus AlbatrossOne" which looks like it could be done now, as in the next thing after neo. Let's see what comes of it.


text:
How are VFR / GAAP approach points selected?
This is kind of an Australia based question.
How are VFR approach points selected and why they are where they are? Also what challenges will pilots face from the GAAP (General Aviation Airport Procedures) approach points to the VFR approach points transition?

The reason for VFR approach points is to provide an orderly arrival procedure into busy or complicated airfields.
I’ll use the example of Bankstown airport, which is the primary GA airfield for Sydney. I have (very crudely) circled the two VFR approach points in green:

Two RN is the approach point when arriving from the south or west. This is identifiable from the air as a large radio tower next to the intersection of two major highways. To the north, prospect reservoir is even easier to see as a dam just south of another large highway. Easy identification from the air is a key requirement for a VFR point, so you don’t get lost.
Why funnel inbound traffic through these points though? Well firstly, much of the airspace over Sydney is Class C and reserved for the heavies in and out of YSSY to the east. You can ask to enter this airspace but chances are you’ll be denied. To the south you also have restricted military airspace over Holsworthy, and to the north-west (mostly out of this map) you have Richmond airbase, two no-go zones. As a VFR pilot you’re not allowed to use GPS as a primary means of navigating, so tracking via these visible approach points greatly helps in not avoiding these areas.
Using approach points also helps the Bankstown ATC. Firstly though I need to clarify that in 2010 all GAAP airports were converted to Class D airspace, which is more in line with international setups. The main change for pilots was that entering via VFR approach points isn’t actually mandatory anymore (unless otherwise stated in the ERSA, which is the case for Bankstown).
Anyway, Class D airspace has ATC but no radar. Therefore ATC relies on procedural separation – that is, pilots broadcast where they are now and where they are going, and ATC makes sure it will be X minutes before someone else says they are on the same path.  So, if two aircraft report passing Prospect within 30 seconds of each other, ATC would probably tell the trailing aircraft to slow down or fly an orbit.
You ask about the challenges of 'transition' but I'm not sure what you mean - these are the entry points for Bankstown, from which it is quick and easy to visually track to the circuit. For other Class D airfields, as I said before, now that they are no longer GAAP you don't have to enter via approach points unless otherwise stated. And if a procedure is stated, AFAIK it will always involve a VFR approach point.


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Descending while turning inbound on a non-precision approach
I'm writing my instrument rating examination soon and one of the questions that I came across was:

While executing a VOR A RWY 01 approach for Alexander bay, 2700ft is reached at 8DME on the outbound leg. The most appropriate action would be to:

level off and turn inbound before descent
continue outbound and descend to 2500ft before turning inbound
turn inbound while descending to 2500ft >>> Correct

Now some background info: Alexander bay's VOR approach outbound leg ends at 8DME 2500ft.
I would say answer 1 is correct, however the answer is stated to be no. 3
Is it because it's a NPA? I was taught never to descend while turning inbound and that it is sloppy flying/poor planning. So this answer doesn't really make sense to me.
I tried looking it up in 8168 and the ICAO docs, but I haven't had any luck.
This is the plate:

Source: caa.co.za (PDF), click image to view

It would help if you can link the plate you are referring to.

Alexander Bay South Africa?

I might have bit off on #1 as well, but I think #3 can be justified.  I don’t have time to dig right now, but I think the answer is that if you are within a published segment of the approach, (including a depicted hold or procedure turn) and have been cleared for the approach, you are then cleared to descend to the next published lower altitude for that segment.  Without seeing the plate you are referring I can’t tell if the IAF you are maneuvering to is at or above 2700, or 2500.  So to PilotDan's point, please provide a plate or more detail so we can help.

Interesting approach, different format than I am used to seeing, but it looks to me from the profile view that if you are heading 187 outbound you need to turn at 8DME, but that you can descend all they way down to 2500 until established inbound at 007.  This interpretation supports answer #3.  (FMI, what are the two altitudes, one bold and one not?)

@MichaelHall the bold is your altitude with current QNH set and the not-bold is your height above ground or altimeter reading with QFE set.

@JohnK Yes, this is in South Africa.

It's because 2500 feet is the minimum altitude prior to starting the inbound descent at 8 DME and you can descend to that altitude pretty much anywhere between ABV and 8 DME inbound, within the protected airspace of the procedure turn.
When you started the outbound leg at 3000, you can pretty much descend to 2500 sooner or later, as long as you don't descend below 2500 until you are 8 DME inbound on final.  So if you find yourself at 2700 at 8 DME outbound and you're ready to turn in, you can keep descending to 2500, leveling at 2500 until you cross 8 DME inbound.
Basically, the airspace between ABV, the 10 mile circle, and 30 degrees of the approach path is the protected airspace with a floor of 2500, and technically, you can do whatever you want within that zone.  Also the altitude is a minimum.. you don't have to descend to exactly 2500, technically (I'm assuming SA IFR rules are similar to NA)


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What is the difference between a non-directional signal and an omidirectional signal from a radio aid?
NDB's are non directional and VOR's are omnidirectional.
What is the difference between a signal that is non directional and a signal that is omidirectional, if any?

Think of an NDB as dropping a small pebble in an otherwise still/static mass of water like a lake. It generates ripples that expand radially from the point in which you dropped the pebble, in the shape of a circle.
If you see the ripples, by judging in which direction they move, you can determine where they come from and all you know is that if you follow that track, you'll eventually find out where the source is. You don't really know (unless you work it out mentally, use a compass and some charts) on which bearing of such NDB you are just by getting the signal (ripples), and it gets even more complicated if you were not flying towards it already
Now, think of a VOR. VORs are fundamentally different as they broadcast 360 different signals (one for each degree of azimuth). Instead of dropping a pebble into a large mass of static water, imagine you placed 360 different colored lasers (I know this may be difficult, just bear with me for simplicity's sake), each one emitting a straight light beam, each color/wavelength on a predictable azimuth from the source. Once you get in the path of one of these (let's say 524 nanometer green), you can instantly tell not only where the source is (it's along the propagation line of the beam) but also you can tell in which exact bearing (relative to the VOR) you'd approach the source if you followed that specific track (called a Radial)
Why is the difference important?
When you're airborne, you are subject to forces other than the basic lift vs gravity and thrust vs drag. There's also winds aloft which almost all the time mean that flying a 300° heading will not equate to a 300° track.  If you were to use an ADF needle to fly towards an NDB in any condition other than completely static air, you'd keep the ADF needle at the 12 o'clock position (flying towards the NDB) but you'd trace a curve over the ground. This is very non-precise way to follow a track.
VOR's on the other hand, let you know exactly where  you are relative to them and by following a specific radial you'd be sure you'd be approaching the VOR from the exact direction you should be approaching from. Hence, airways built on VOR's are so very much more precise than NDB-based ones
Now regarding specific differences in the signal:
1.- NDB signals are usually in the KHz wavelengths while VOR and other directional beacons transmit in the Mhz ranges (right below the air band for radio comms, by the way)
2.- As stated above, the NDB broadcasts a single ommnidirectional signal. Directional beacons will (excuse me for the over simplification) transmit slightly different frequencies on each azimuth. No two azimuths will be on the same frequency, so for each VOR each frequency uniquely identifies a specific radial.
3.- In real life, radials are not completely straight and non-overlapping "lines". They have a cone-shaped dispersion pattern (just as a laser produces a dot of different diameter as travel distance increases) so being "on the radial" still has some margin of error and thereforce VOR-based airways are still a couple of miles wide from the centerline... if you are 100 miles away from a VOR and "centered" on a specific radial, you may still be offset by as much as a mile. As you track the radial inbound, your offset becomes smaller until you overfly the VOR

Excellent answer, but the practical effects you describe are still heavily weighted towards typical GA instrument limitations imposed by having separate DG, ADF and OBS.

Yes, that's true. Better equipped aircraft, with better systems integration, are able to mitigate or eliminate those effects, which is partly why we have RNAV RNP and GNSS with inches-level precision. However, I thought it might be useful to consider "the basics" and work our way from there :)

I’m not talking even about RNAV, simple needles on a compass card render the ADF and VOR indications you describe moot.  For those not schooled in ADF homing or OBS operation it gets confusing.  The original question was really about the signal difference anyway, so perhaps even touching on cockpit indications is unnecessary.

Point taken. I agree. I do, however, believe that while it may be unnecesary, it makes it a little bit easier for the un-initiated (which I can't tell if the OP is or not) to understand. But yes, I went a little overboard on that

So is there a discrete jump in the VOR signal, once per degree of azimuth?  I'm guessing not.  I'm guessing it is more of a continual kind of thing.

It kinda is, but for the sake of simplicity and brevity, the explanation is very generalized. Each radial does in fact have a slightly different frequency but as their dispersion cones overlap (they are separate at the source, but as they propagate, two contiguous radials overlap a bit) the whole VOR signal works, just like an ILS, as a single one and the onboard equipment determines phase-shifting to pinpoint its location relative to the VOR. The math is beyond me at this point in my life though :)

That's a brilliant answer, thank you @JavierLarroulet. I just wonder why the question in my paper assumed that omnidirectional and nondirectional are the same thing.

"Omnidirectional" may not be the most descriptive word for the actual radio signal emitted by a VOR, as contrasted with the signal from a NDB.  We could note that a VOR and a NDB actually both share the characteristic of broadcasting in all directions, as opposed to the narrow beam emitted by a landing approach aid, or the characteristic pattern emitted by the "four course" radio ranges of long ago.
But as for the difference between the radio signals emitted by a VOR and a NDB--
Think of the signal from a VOR as a collection of beams radiating out from the transmitter.  Each beam carries data: it is encoded with the azimuth from the station.  You can easily find more information about how this done: in actual practice the azimuth information is encoded in a phase shift between two different signals.
With a NDB, we also have beams radiating in all directions from a transmitter, but all the beams are the same.  That's why you can only get a relative bearing from a NDB.

Well, by combining it with the heading from compass, you can easily get absolute azimuth from relative and vice versa, so you can get the same information from either. However, measuring the phase difference in VOR is more accurate and less affected by environmental factors, making it practical to fly along specified radial rather than just generally towards the beacon.

cloverleaf pattern

To add to @quiet flyer's answer, the practical result is:
With Non Directional you only know the radio aid's bearing from the aircraft's longitudinal axis and not the aid's geographic orientation to the aircraft's position (until you mentally work it out).
With Omnidirectional, you only know the aircraft's geographic orientation to the radio aid's position; that is, its bearing from the radio aid itself (but not the range), but not its bearing relative to the aircraft's longitudinal axis.
Put another way, with an ADF you only know whether it's in front, behind or off to the side at a given azimuth of the airplane at any moment.  With an OMNI you only know whether the aircraft is north, east, south, or west (or any azimuth angle in between) of the navaid itself.

This answer is correct, but it doesn’t add clarity unless you understand the limitations of instruments common to general aviation.  For example, if you had a slaved RMI compass card with a #1 ADF needle, and a #2 VOR needle, and you tuned your receivers to co-located NDB and VOR signals, both needles would point the in same direction on the compass card, (magnetic bearing to the station) regardless of aircraft heading.  In this case there would be no practical difference in the information displayed, regardless of how it is derived.

As a random aside, we can note that it would surely be possible to build a direction-finding function into a VOR antenna /receiver system, so that it would indicate the station's relative bearing from the aircraft, as well as the station's actual azimuth from the aircraft, even without incorporation of a slaved RMI compass card.  This would be "doing things the hard way"...

But why "do things the hard way" when a simple and elegant solution exists?

Yes but I think that goes way beyond the basic question.  Anyhow, nowadays I navigate my own plane using a program called Naviator installed on my phone.  I don't even bother with the Garmin 295 in the plane any more and haven't used a VOR in eons.  My plane has a bubble canopy so the phone's or tablet's GPS works fine for VFR wandering around in nice weather.  I carry a paper map as a backup.  Pretty amazing that a guy in a Paramotor or ultralight with a cell phone has probably more navigation power than a 747 in 1970.

I agree.  I use my iPad for pretty much everything.  But, you have to agree that an RMI is way better than separate DG, OBS, and ADF.

My understanding was that a VOR was not a series of different signals being sent out in narrow cone shapes. I understood VORs to be just a few signals being sent out to include the station identifier, an optional voice signal, a reference signal, and a variable phase signal, as well as some other discrete signals.
The navigation part is done by sending out a reference signal that is sent out in the same phase in all directions. We will say this signal has a phase of 0. Then, a highly directional antennae sends out a very concentrated signal as it is revolved around the center of the antennae array. As the antennae revolves, it rotates the phase of its signal in correlation to its angular position in its revolution. In other words, when the antennae is broadcasting in the direction of Magnetic North, its signals phase is 0. As the antennae moves in its clockwise arc, the signals phase will shift 1 degree of phase for each degree of movement. By the time the antennae’s signal is broadcasting Magnetic East, the signal’s phase has shifted 90 degrees. And so on, and so forth. The receiver in your Nav radio can receive both the reference signal and the variable signal, sense their phases, and compare the difference. The difference in the phases is the Radial on which you are located.
As far as NDBs, think of old pictures you’ve seen of pre-jet-age planes. The older ones had loop antennaes on the top and/or bottom of the fuselage. These loops were used to pick up the signal of an NDB. The NDB just sent out one navigation signal in one phase. But, by rotating the loop antenna, you could tell which direction the signal was coming from by sensing whether it got weaker or stronger as you rotated the loop. There was a second simple sensing antennae on one side of the loop antennae to ensure that you did not make a 180 degree mistake.
The DME portion of the VORs and NDBs are a whole different baileywick. These are call and response devices on a whole different frequency than the VORs and NDBs. Your DME radio sends out an encoded and timed call signal. The DME station sends back a coded and timed response signal. The DME calculates the time difference to determine the slant range distance. The bandwidth space on these station is finite. That is why you will see in your charts and chart supplements requests to turn your DME off when entering certain terminal approach areas like DFW. The Maverick VORDME (TTT) only has so much bandwidth. The FAA wants to reserve its use to those airplanes that really need it. Especially those at or above FL240.
ILS and Glideslopes are a unique kettle of fish. Each has antennae that send out two different frequencies. These antennae were highly directional and concentrated. The ILS has antennae that are side by side. So the two frequencies are effectively the left frequency and the right frequency. If your Nav radio is picking up both frequencies equally, you know that you are right in the middle of the ILS. If your Nav radio is picking up one frequency more than the other, you know which side of the ILS you are on. The Glideslope works the same way except the antennae are one on top of the other. Now, most of the antennae are uni-directional, but not all of them are. Some are still bi-directional. And, since the antennae are fixed in place either side by side or one on top of the other, so are their signals on the back side. So, if you have an ILS that is sending a signal to the North, its right-hand signal, as you face the antennae from the North, is on the West side. Its left-hand signal, as you face the antennae from the North, is on the East side. But, if you face the antennae from It’s backside on the South (its Back Course), its Right-hand antennae is still on the West side. Which is your left-hand side. Its Left-hand antennae is still on the East side. Which is your Right-hand side. People call this reverse sensing. It’s really true sensing. We are reversing it in our heads. Be careful on the backcourse to get this correct. The same is true of the glideslope. But it is not as prone to confusion because top is still top and bottom is still bottom. Up is still up and down is still down. And, the backcourse signal is pointing 3 degrees into the ground for the most part.
Oh, and to answer the original question:
Non-directional means that the signal is broadcast out everywhere without any focus on direction.
Omni-directional means that the signal is broadcast out in every and all directions, but with focus on each direction.


text:
How to know that single engine aeroplanes are not allowed to depart on the TETAN SID?
I'm studying for my IR exam and came across another tricky AIP question.

Which of the following statements about the TETAN 1A standard departure chart are correct?

Single engine aeroplanes are not allowed to depart on this SID >>> marked as correct
Aircraft equipped only with RNAV (GNSS) avionics equipment may not fly this procedure
If the SID is in force, an aircraft that cannot comply with the minimum climb gradient on the chart may not take off
The altitude at which Cape Town Radar control must be cancelled will not be confirmed in the ATC departure clearance

I would say #3 seems more likely because of the terrain. However, it says that that if an aircraft cannot comply with the climb gradient, it just needs to notify ATC.
As for #1... how would one determine that? I've had a look at the TETAN C and TETAN B Dep charts and they don't indicate anything about being single engine or multi engine aircraft limited either.
I'm located in South Africa and we use ICAO as a reference for most of our regulations.

(Click here or on image to view full size, or here for the PDF)

Well it can't be 3 because it just says if you can't comply, contact ATC, not that you can't depart.  You just can't accept the SID in your clearance.  As to why there is a single engine ban, I'm a bit stumped; does that mean you can't depart in a single engine turboprop?  Did you look around the set of charts for some other comments or restrictions?

Your link throws a 500 for me

Welcome to aviation.SE! Just as a general comment, if you ask "is X allowed?" then you're most likely asking about regulations. If so, please always tell us the specific country or regulations that you're asking about. That helps the community to give you better answers.

As I read the question, I don't understand why you have "1... >>> Correct". Is this a typo? Are you trying to imply that 1 is correct?
ATC is allowed to relax constraints in a SID that are due to traffic/airspace, but it is not allowed to relax terrain restrictions. It can, however, reroute you to avoid terrain.

@MarkJonesJr. I have no. 1 marked as correct according to the material that we've been supplied with to prepare for the exam. It is the most correct answer according to the CAA. I do agree with what you are saying but they mark it as incorrect if you choose this answer no. 3 in the exam.

I don't really know, but the only reason I can imagine is that the climb gradient due to terrain implies—by some general rule—climb gradient with one engine inoperative, which would then single-engine aircraft obviously not satisfy. Can you try looking for something like that in the general materials?

As a complete guess, is there a regulation that means that single-engine aircraft may not be routed over water by ATC? The climb rate is certainly achievable in many single-engine aircraft so I don't know why that would be a consideration.

@Skydemon, is there also a text part of the AIP for this SID? I am asking because in a lot of AIPs I came across this contains the RNAV coding and other details/restrictions.

In the USA test preparation products use questions from the FAA but the answer set is created by the publishing company because the FAA does not make the official test answer key public. So it is not uncommon to find answers sets in preparation products that are poorly worded or just not accurate.


text:
What would be the "minimum" number of flight crews per aircraft in a small low cost airline?
Let's say we own a small company flying 10 airplanes (A32X/B73X/A220/E-Jet) in high density low cost environment. Leaving all other costs outside the equation, What would be the minimum flight crew members per ac needed to make this hypothetical company profitable? Let's assume we want to keep flying our fleet as much as possible (...say aircraft utilization of 8 hours).

Related (possible duplicate): What is a typical ratio of airline pilots to aircraft? and How many pilots does an airline hire for each new plane they purchase?

Are you asking about pilots only, cabin crew, ground crew, or all of them?

@Bianfable This is related to pilots only. I read similar questions but this one is specifically asking a minimum number of pilots in a low cost and small size company operating single aisle ac in medium to short haul. I heard the number of 4.3 and sometimes 5 pilots but I don´t know how is this calculated.

@GdD Only pilots

There is a world of "it depends" involved here, but as a bare minimum starting point:
To get high utilization of each aircraft, you'll need to fly it longer than one crew can stay on duty.  So each day you'll need at least two crews: one to start in the morning and fly until early afternoon, and then another crew to fly from then until the airplane is done at the end of the day.  So each day, nominally, four pilots touch the airplane.  (Yes, there are times you can use just one crew for the whole day - start in California at 0800L and end 10 hours later in New York at 2100L - and times you'll end up with a third crew... flying three 5-hour legs would require that, but let's use two crews/day as an average.)
A 3-day trip is pretty common for domestic crews.  Long-haul international gets longer trips, but with augmented or double crews, so use the short-haul case to keep things simple.  So each week, an airplane has crews A and B for the first 3 days (an AM crew and a PM crew), crews C and D for the next 3 days, and there's one day left.  If everybody works a 3-on, 3-off schedule, those 4 crews can man that aircraft all month long with a total of 8 pilots.
But even in an ideal case, that's not enough pilots.
First, you need reserve coverage, so that when stuff goes sideways, there are pilots on call who can fill in the gaps: weather causes a crew to time out, or somebody gets sick, or whatever else happens that you need another pilot.  Depending on the operation, reserve coverage can be 10 to 20%, or sometimes higher.  FedEx and UPS, as I understand it, have very high reserve coverage ratios because of the nature of their operation, but that's not what the OP posits, so let's say 20% coverage.  Now 8/aircraft is up to about 9.6 pilots / aircraft.
Also, 3-on/3-off can get tiring, and may push crews up against limits on block hours and FDP (see FAR Part 117 for US limits on such things), and something closer to 3-on/4-off is probably more common.
Beyond that, some number of pilots just aren't available in a given week: an individual may have recurrent training, or be on vacation.  At a startup, there won't be a great number of pilots out on long-term medical, but as the operation grows that happens.  Likewise, nobody probably has much vacation in the first few years of a new company, but training is required.  And since pilots can not fly with a head cold or other "minor" illnesses that an office worker can work through, there are inevitably some number of sick calls.
All of that adds up to something in the vicinity of 11-13 pilots per aircraft for a lean operation, and somewhat more than that as things get more complex and more mature.
There are ways to reduce that number, like "we don't fly on Tuesday or Thursday" and "we don't overnight crews away from the hub" -- both of which can be done; I don't have hard numbers for how much that reduces the ratio.
But as a general ballpark, 11-13 pilots per aircraft is a good starting point.

Nice analysis! Your result agrees quite well with the two single-aisle only airlines in the table in this answer. For the larger airlines with twin-aisle aircraft in their fleet it seems to be somewhat higher, around 16-17.

@Bianfable Thanks!  Yeah, the twin-aisle jets get into enough long haul flying that basic assumptions for them look different, with augmented or double crews, layovers of 24+ hours, and so on that their ratios will be pretty different, which raises the ratio of a United or Delta with a mix of both narrow and wide body jets.

If you're searching for optimization opportunities for the flight attendants, the FAA has multiple requirements and guidelines for flight attendants. (I found the requirements of the FAA first and clear to understand, that's why I explained them)
Crew Amount Requirements
To operate an aircraft the amount of crew is dependent on seat capacity:

19-50 Passengers require: one flight attendant
51-100 Passengers: two flight attendants
over 100 Passengers: two flight attendants plus one attendant each for every unit (or part of unit) of 50 Passengers.

§ 91.533 Flight attendant requirements
So e.g. for a full economy class A320 with 195 Passengers you are required to have four flight attendants on board.
**
Rest Period
**
To calculate your needs of personnel, you also have to take the minimum required rest period into account.

normal duty of up to 14 hours: 10 consecutive hours rest [(2) and (3)]
long duty between 14 to 16 hours: 12 consecutive hours rest plus one additional flight attendant over minimum requirement [(4)]
very long duty between 16 and 18 hours: 12 consecutive hours rest plus two additional flight attendants over minimum requirement [(5)]
extremely long duty between 18 and 20 hours: 12 consecutive hours rest plus three additional flight attendants over minimum requirement [(6)]

There are further additions under subsections (7)-(9).
§ 121.467 Flight attendant duty period limitations and rest requirements: Domestic, flag, and supplemental operations.
With this information, you should be able to calculate your needs.

The question was about flight crew, not cabin crew.

Ah, RIP, perhaps its interesting anyway :c

Very good answer anyway


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Are there significant differences between EASA and FAA when teaching jet aircraft performance?
Both regulatory bodies did a great effort in regulation harmonization, but I'm looking for any significant differences (if any) in how is taught the subject from the pilot point of view, i.e. differences in landing and takeoff performance terminology and data calculation.

Define “significant”.  I assume you use it to means differences where the outcome or decision made would be different.  Calculations go back to physics so there should be no difference in calculation results.

I suspect you’re going to find much greater variations between manufacturers than between regulators. US regulations don’t say much at all about performance (or how to teach it) other than that you must adhere to the limitations in the AFM/POH.

@Jim. Of course physics is the same. Significant in this context means any differences in climb gradient, factors, flap setting for takeoff or go around, takeoff or approach speed calculation, etc. Also distances and allowances for dry and wet runways (even definition of contaminated runways).

@StephenS. As an example, the calculation for unfactored landing distances are based on Regulation 135.385 (b) (FAA ) and CAT.POL.A.230 (EASA). Here are some differences in terms and definitions and also in how to calculate those factors in wet and dry runways.

Regulators aren’t the ones that determine flap settings, takeoff/approach speeds, or dry/wet runway performance.  That’s done by the aircraft manufacturer.


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Could compressor stalls be caused by air in supercritical conditions?
It is likely that in compressors of jet engines appear conditions that cause the intaken air to become supercritical, cf Is the air in a jet engine supercritical? .
Is it possible that supercriticality of the fluid (air) is one of the causes for compressor stalls or compressor surge ?

The answer to your question:

Is it possible that supercriticality of the fluid (air) is one of the causes for compressor stalls or compressor surge ?

...is definitely no.
Stall is caused by an excessive angle of attack of the incoming air, relative to the blade. It can happen on the first stage (first row of rotating blades), where the pressure is no where near high enough to be supercritical. Surge occurs when the stall is very severe, so it’s the same phenomenon, just worse.
While the likelihood of a stall occurring is determined by measuring the total pressure distortion across the compressor face, it is the angle of attack that is the critical factor. Regions of low total pressure indicate the flow is likely to be turbulent in these regions, and therefore the angle of attack is not steady, and can exceed the ability of the air to follow the surface of the blade. Once it separates, it becomes turbulent, which means the blade does not perform its function of compressing it well. This is a big problem, as the compressor is trying to get the air to flow “uphill”. Stall generally starts as a phenomenon involving a few blades on one or more stages. It can then develop and cover a large section of the compressor. If the axial velocity of the flow through the compressor starts to vary, faster and slower, then faster again, it is called surge. The engine “chugs”.

stall can also be caused by ingestion of objects so i think the root cause of stall and surge is a non-equidistribution of pressure, flow velocity, ... on the surface of the first stage and following stages of the compressor that also occurs in a high angle of attack

@ralfhtp No, if the distribution of pressure and flow velocity is uniform, but the pressure gradient is too large/the flow too slow, it will also stall (all at once). This is basically what could happen in pre-FADEC engines if you added power too quickly—the pressure would increase too fast, the flow would slow below what is appropriate for the compressor speed, the compressor would stall, and the engine would usually flame out as it burned out all the oxygen without fresh air coming in. The reason is really that the flow is too slow, which is usually because the pressure gradient is too high.


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Is the air in a jet engine supercritical?
Nitrogen has a critical point of 126.2 K and 3.4 MPa (34 bar). Air contains 75 % of nitrogen

For example, nitrogen has a critical point of 126.2 K (−147 °C) and 3.4 MPa (34 bar).

source : https://en.wikipedia.org/wiki/Supercritical_fluid

In a jet engine pressures often exceed 34 bar ( as seen on Wikipedia, where for example the GE9X is shown to reach 60 times ambient pressure ), the critical temperature of nitrogen is exceeded at room temperature. So is air ( fluid ) in the high pressure regions of a jet engine supercritical ?

Maybe at takeoff, definitely not in cruise
You have to consider that atmospheric pressure drops with altitude. At FL360 you have ~0.226 bars. Even multiplying with the GE9X's pressure ratio of 60, you get ~13.56 bars, well below the 34 bars needed.
Takeoff might be a different situation, since the engines are operating at full power with ambient pressure ~1 bar, this could lead the high pressure compressor stages to operate in supercritical conditions. Note though that the combustion that immediately follows changes significantly the chemical composition, and thus the critical point might be altered as a consequence.
As engines are optimized for cruise conditions, it is highly unlikely that supercritical conditions have extensive design consequences, unless they would render takeoff impractical.

yes at first i was not sure about the design consequences however jet engines have to be able to handle vast amounts of water intake ( heavy rain , ... ) and they are. The properties of a supercritical fluid compared to a gas differ strongly, supercritical fluids are in a state between liquid and gas. i now wonder if supercriticality is related to compressor stalls / surge, maybe i ask a new question ...

removed Accepted Answer because second answer is contradictory

@ralfhtp sure, no problem, but I don't think we are contradicting each other. I said "might" because I was not sure, ymb1 says "that's not a maybe, I know it is not". But the question is yours, if you need more clarity from the answers you can obviously ask for it.

i am also not sure because if can find no papers about this...


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Is there any regulation that force(or recommend) to change flight number in case of flight delay?
I have a question about a flight number of delayed flight.
I think that if the daily flight(e.g. OZ9609) is delayed until the following day, there are two flights with a same flight number.
In this case, I think operating two flights with same flight number could cause confusion to passengers and systems related with operating the airport.
So I want to know there is any regulation that force to change a flight number if situation like this case occurs.

The flight number that passengers see and the flight number pilots and controllers use are quite often two different things. The FAA doesn't really regulate the passenger side of that, so this may be of topic

@RonBeyer Why would something be off topic just because it is not an FAA regulation? This is not faa.stackexchange.com

You might be interested in Do flights of a specific callsign always depart and arrive at the same airports? (Full disclosure: The accepted answer to that question is my own.)

Welcome to aviation.SE! For questions about regulations, we generally need to know which country or regulator you're asking about. I think that's an Asiana flight number, are you asking about South Korean regulations?

@Pondlife In fact, I tried to find my country's but I could not find any regulation related to my question. So I would like to find the regulations of U.S. or other countries or international standards(e.g. ICAO, IATA, etc.) if there exists.

@J.Hougaard it is not faa.SE, but we have explicitly said that passenger-side questions are off-topic here and should be posted on Travel.SE. That's what Ron was getting at. You might disagree and think this is not a passenger-side question, but that's a different point.

@Federico, if the correct answer is "no, there is no such regulation" or "passenger flight numbers are different from operating flight numbers", that doesn't render the question invalid or off-topic. It just means that the answer is very short.

@bogl I am not claiming that the answer is off-topic, I am explaining what Ron was saying.

No, there's nothing that forces a flight number change besides avoiding multiplicity
In general terms, it may be an unwanted complication to have two flights on the same route, which operate in the same day, with the same flight number. This is specially troublesome with delays in which the first flight ends up departing on a different date because of RPLs (Repetitive Flight Plans) may be on file with aviation authorities.
Take for example Delta's flight from Santiago, Chile (SCL) to Atlanta. Operates daily as DAL146 departing some time around 10pm local Santiago time and arriving into ATL early in the morning of the next day.
There is a RPL filed with the Chilean aviation authority with the general flight info (daily operation, always same aircraft time and scheduled time of departure, generally the same route, and flight level, etc). This eventually gets overriden every day by the final operational flight plan that takes the most recent conditions into account, but for tactical reasons, the RPL is filed days or weeks in advance.
Now, consider what happens if today's DAL146 is delayed for whatever reason and is now scheduled to depart at 03am local time tomorrow (a 5 hour delay). IF the flight departed as DAL146, there would be a lot of reasons for confusion:
two flights departing the same city, same route and same flight number on the same day. This messes up a little bit with airport systems, air traffic control and the own airline's systems as the first(delayed) departure would cause the future flight to be seen as "departed on time" hours before its actual departure
For this reason, cases like this usually end up with the rescheduled flight getting a new flight number (you may see DAL146A, DAL146Z or even a completely new flight number, depending on internal airline's protocols) so even if the two "identical" flights end up departing on the same date, there would be no confusion.
Another reason for this to happen is multi-leg (from A to B to C) flights in which one leg (A to B) is delayed but the next leg (from B to C) can be operated  by another aircraft in order to minimize passenger disruption and help with the fleet positioning. Cases like these, if left unhandled, could lead to more than one flight using the same flight number at the exact same time, which is something that simply cannot be had, so one of the flights will get an alternative flight number to prevent the conflict

An airline's flight number for commercial purposes is not necessarily the same as the identifier the aircraft uses for air traffic control purposes. For example, British Airways flight may show up on a flight tracking web service as BA### but air traffic control identifies them as "Speedbird ####" where the number may or may not be the same as the airline's commercial flight number.
So, to answer your question, there is no FAA regulation that forces the airline to change commercial identifiers. The ATC identifiers are set when the dispatcher files the flight plan, and they must be unique to ATC. The system will reject it if the identifier is in use, or will be during any period of flight.

The fact that ATC callsigns are different from the flight number is only due to the need to translate the airline code into something more easily spoken over the radio than a series of letters in Phonetic. ATCOs see the flight number as BAW123 (ICAO code) in scopes and flight strips, but saying Bravo-Alfa-Whiskey One-Two-Three takes too much time. Speedbird is what all british airways flights will be called over the radio


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Passengers Weight configuration
Well as usually when you fly in an airplane they Determine the luggage weight but how do they calculate the airplane weight "necessary so the plane doesn't get overweight" when they don't know the passengers weight and how do they deal with obesity (after my respect to all differences between people)

related, if not duplicate: https://aviation.stackexchange.com/q/27921/1467

It depends on the size of the plane. On a company-operated small bizjet, I've known situations where the pilot comes into the passenger cabin before takeoff and says  "OK, some of you need to change seats, I want to see the heavy guys sitting in the front seats and the light guys at the back".


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Why does airport security use numbered baskets for my carry-on stuff to be x-rayed?
I recently flew from TPE and noticed that there, the baskets for the carry-on stuff that gets scanned are numbered:

I can't think of any reason why this would be useful. Can anyone explain?

@ymb1 Was considering that, but decided to post here, because first, the question is more about the operational aspect (off-topic for the travel SE, because it does not affect my experience as a passenger), and second, there is an airport tag.

There are a few reasons that they would mark baskets.
Identifying Baskets
Firstly, as a previous answer stated, numbering baskets ensures that the security staff can correctly identify baskets. If they need to pull a basket off, they can tell their colleagues the number, and their colleague can pull it. The alternative would be to remove it themselves. As security staff can be on either side of the scanner, this is much more efficient. A staff member may notice a suspicious person load a basket from one end, and they can request a staff member to staff on the other end.
Connecting Baskets to Passengers
The more important reason for numbering baskets is to provide a way to connect baskets to passengers. Say the security staff finds something concerning in a bag, and they ask "whose bag is this?" - the terrorist/smuggler/criminal is not going to walk up and claim ownership! With the numbering system, cameras can be used to connect particular basket numbers with the person seen putting the items in. Next time you walk through TSA, look directly above each conveyor belt for the scanners - there is typically a camera with a topdown view.

But with camera you don’t need number to connect passenger to the basket

It's more meant in the fact that there are agents who are (supposed to be) watching via those cameras, and they can contact on radio, and say, "Check basket #4, owned by the guy in green jacket". I say "supposed to be", because it's been seen quite a few times recently that security personnel have been shirking some of those less supervised duties.

It makes it easier to pull a basket from the line. Its likely that those numbers show up on the X-Ray screen so that if the person watching sees something that needs inspection they can say

pull basket number 18

instead of,

i think thats the one to pull, it was 4 baskets after the one with the shoes and the laptop.

There are also ones are available now with RFID tags inside for a similar reason.

That's also my suspicion. Any source? Or, more interestingly, any insight on how it works that the numbers appear on the screen?

Metallic or similar label with cut outs for the numbers. The metal will block xrays, while the cut out let's them trough.


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Isn't a "land based navy bomber" just an air force bomber?
This wonderful article on the Mitsubishi G4M Betty has the following quote about the Mitsubishi G3M Nell:

the G3M was already considered the best land-based navy bomber in the
world

What in the world is a land-based navy bomber?
Is it not just an air force bomber?

land based navy personnel are just an army soldiers?

"Land-based" refers to where the plane takes off and lands.  No suggestion was intended that it operated exclusively or mainly over land.  Which is not to say that there could never be a "navy" aircraft that actually operated not only from land, but primarily over land as well.  Such examples undoubtedly exist.1
Would you consider the Consolidated PB4Y Privateer to have been an "air force bomber"?2
Would you consider the B-25 bombers involved in the "Doolittle raid" to have been "navy bombers"?
The Imperial Japanese Navy and the Imperial Japanese Army were very competitive rivals during World War II (see Saburo Sakai's famous book "Samurai!".)3  And there was no Japanese Air Force at this time.
Footnotes:

Examples would include many trainers, such as the N2S and N3N.  Note however that with a few exceptions (e.g. Doolittle raiders), most navies would probably not look favorably upon non-navy aircraft operating from aircraft carriers!

Admittedly a slightly imperfect analogy because the PB4Y was specifically a patrol bomber, which is more of a dedicated maritime role than the G3M was likely designed for.

Speaking of the book "Samurai!", note that Sakai was a navy pilot flying navy aircraft, yet all of his operational missions were from land, and many involved missions against targets on land.

The US Navy E-6 Mercury TACAMO aircraft are both land-based and also fly primarily over land; the Navy P-3's and P-8's are land-based but fly extensively over water.

The terms "Navy" and "Air Force" refer to distinctly different branches of military service.  (The United States also has the Marine Corps, Army, Coast Guard, and Space Force)
Most branches of most militaries operate aircraft of one type or another, either to conduct their primary mission, or for support.
For example, in the United States:

The Navy and Marine Corps operate ship based aircraft, (fixed and rotary wing) as well as land based.
The Airforce and Army are exclusively land based.
The Coast Guard operates helicopters from its ships, plus land based fixed wing transport aircraft.
The Space Force, well... they are kinda new still and we don't have any Star Wars type craft just yet.

Whether or not a certain airplane operates from ship or shore doesn't change the branch of the military that has ownership and control over the aircraft assigned to it.

A fine example of what happens to an aircraft when the Air Force, Navy and Marine Corps slap their specs on a design, is the F-35.

The US Space Force does operate aircraft.

@JonCuster, my remark was tongue in cheek, but since you brought it up, what kind?

Well, the X-37 might count. I guess I had assume the Space Force would have some aircraft for transport between bases. Or does the USAF supply those as needed?

@JonCuster,  Guessing the USAF supports, but they could have a few C-12s.  I wouldn’t count the X-37 as an airplane - From the limited info I’ve seen it’s functionally more of a reusable satellite.

Soviet Naval Aviation also flew land based bombers - Backfires & Badgers - for missions involving maritime reconnaissance and interdiction.

@MichaelHall - agree. The Air Force vs Space Force budgets are still in flux as various things get transferred over. Might take a while to settle things.

Another distinction between a "naval aircraft", and an aircraft designed for only land-based operations, is that naval aircraft are built to handle carrier takeoff and landings.
That means they have probably have stronger landing gear, carrier landing hooks, maybe even folding wings.
Related: Why did the RAAF procure the F/A-18 despite being purpose-built for carriers?

Well -- re "naval aircraft are built to handle carrier takeoff and landings" -- see my answer re PB4Y.  So, not a "naval" aircraft?

You aren't technically wrong, but the answer is very incomplete.

The US Navy E6B and P8 are examples of a land-based Navy aircraft. Definitely not certified for carrier ops.   https://en.m.wikipedia.org/wiki/Boeing_E-6_Mercury


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How do turbofan internals handle the load from the thrust?
To my understanding, turbofan engines get the majority of their thrust from the fan in front of the engine, up to around 70-80% while the core flow generates around 20-30%.
Forward movement is achieved by utilizing Newtons 3rd law - by moving masses of air backwards and thereby moving the aircraft forward.
The thrust will first push the engine forward, and then the engine mounting, and then the wing and the aircraft as a whole - but how exactly is the load from the fast moving thrust air taken up by the engine internals?
As wings generate lift, they bend to a varying degree. Do the fan and engine internals, e.g compressor and turbine blades bend as well? I can imagine the fan taking up a tremendous amount of thrust load and wants to leave the engine and travel forward out the housing.
How much of this is true? Any input or suggested reading on this is highly appreciated!

The thrust is generated at the fan and compressor blades, as these are the components which push the air back. It is then carried through their attachment disks to the shafts and from there to what are called thrust bearings. A simple thrust bearing is a fixed flange against which the shaft pushes, but of course they are usually a lot more sophisticated. Thrust can also be generated by careful shaping of the core exhaust duct, but on a subsonic turbofan it can at best be only a small amount. The force is then transferred through the engine's structural framework to its mountings and thence to the airframe itself.
Yes, everything in the engine will deform a bit under load. Luckily the load paths tend to be short, but it still has to be made stiff and strong, with as you say the mounting carrying the full thrust. Extensive use of high-strength materials is made in the engine to keep deformation down to manageable levels. Varying amounts of thermal expansion of different parts also take place. These combined movements necessitate the designing-in of mechanical tolerances, to prevent the binding of moving parts or local build-up of mechanical stresses.

The rotating parts of the engine, including the fan, are supported by bearings which allow them to turn with a minimum of friction. Those bearings also transmit the lift force generated by the rotating fan blades to the casing of the engine, which we call the thrust of the fan.


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What are these protruding elements from SU-27's tail?
I recently saw in person SU-27 belonging to the Ukrainian Air Force. I spotted some protruding elements from its tail and I'm wondering what are they called and what do they do?

I believe that these are static discharge wicks.  You can see similarly sized/shaped wicks on a 1/32 scale models of the SU-27, as discussed here, but, more importantly, you can find them labeled with number 53 on the cutaway drawing below.  The label is written as Разрядники статического злектричества, which, according to Google Translate, is "ESD arrestor" (or static discharge wick).

These are distinct from the shape of the wicks found on most GA aircraft in the US (just a straight stub of wire, potentially with some insulation around the base), but, as discussed on Aerospaceweb, the shaping of the wick (particularly the inverted cone shape shown here) contributes to increasing the gradient in the electric field from tip to base of the wick.  This encourages accumulated charge to migrate back into the atmosphere from a controlled point and decreases interference with on-board electronics and radio systems.

These are indeed static discharges as can be seen in this cutaway (number 29)

Thanks!  I've added it to the answer.

ah, the good old days when aviation magazines has real content

Oops -- noticed that the cutaway that was up for an SU-35.  Corrected!